A dynamic gain equalization filter (DGEF) is a device or arrangement that is useful for controlling optical WDM channel powers, especially at the dispersion compensating module (DCM) port of an in-line amplifier (ILA) to provide the desired spectral flatness of the output channels. An example of a DGEF is described in U.S. Pat. No. 6,212,315 by C. R. Doerr et al. titled “Channel Power Equalization for a Wavelength Divisioned Multiplexed system,” filed on Jul. 7, 1998, which is hereby incorporated by reference in its entirety.
FIG. 1 depicts a block diagram of a DGEF 100 for controlling channel powers in wavelength-division multiplexed (WDM) systems. In the DGEF 100 of FIG. 1, a decrease in attenuation range can be traded for a decrease in insertion loss. The WDM signal channels enter a coupler 102 from a left port 101. The coupler 102 splits the WDM signal into its signal components which are sent via upper and lower arms, 103 and 104 respectively. The output of the upper 103 and lower 104 arms are recombined in the second coupler 105 having the same splitting ratio as the first coupler 102. The upper arm 103 is a simple waveguide; while the lower arm 104 includes a wavelength selective phase shifter apparatus 104a comprising a demultiplexer 106 coupled to a multiplexer 107 via an array of programmable phase shifters 108. The number of phase shifters equals the number of channels in the WDM signal. In this illustrative example, the array of programmable phase shifters 108 includes four phase shifters; each programmable phase shifter is a device whose effective path length can be controlled externally via a control lead 111. Therefore, the phase of certain frequencies can be selectively varied to produce the desired output signals.
In an add/drop mode, port 121 is the add port and port 122 is the drop port. To throughput one or more wavelengths (or channels) from port 101 to port 109, the phase shift of phase shifter 104a must be 180 degrees for the throughput wavelength(s). Thus, if more than one wavelength (or channel) is to be throughput the phase shift of phase shifter 104a must be 180 degrees for each of those wavelengths. In contrast, to drop or cross-connect one or more wavelengths from port 101 to port 122, the phase shift of phase shifter 104a must be 0 degrees for that wavelength. This means that wavelength does not appear at port 109.
If more than one wavelength (or channel) is to be dropped the phase shift of phase shifter 104a must be 0 degrees for each of those wavelengths to be dropped. This cross-connect mode also enables a new wavelength (or channel) to be added at port 121 and appear at port 109 along with the throughput wavelengths from port 101. If more than one wavelength (or channel) is to be added, they are added at port 121 and appear at port 109. The phase shift of phase shifter 104a must be 0 degrees for the more than one added wavelength. Due to its symmetric nature, one can make a reflective arrangement by cutting the device in half with a mirror placed along the axis of symmetry.
The transfer function of the DGEF 100 is reflected by the following equations:EOUT/EIN=R−(1−R)ejφ√{square root over (T)}POUT/PIN=[R−(1−R)cos(φ)√{square root over (T)}]2+[(1−R)sin(φ)√{square root over (T)}]2=R2+(1−R)2T−2R(1−R)cos(φ)√{square root over (T)}  (1)
where EOUT, POUT and EIN, PIN represent the electrical field and power of the complex envelope of an optical signal at the DGEF output and input (109, 101) respectively;
R reflects the coupler ratio (Rε[0,1]);
T is the power transmission of the phase shift section 104a, where (Tε[0,1]); and
φ is the per channel relative phase change induced in the lower arm 104.
When T is known, selection of an appropriate value for R offers a trade-off between the dynamic range and the maximum transmission of the DGEF 100.
Unfortunately, it is a challenge to find a cost-effective solution for an in-service upgradeable in-line amplifier (ISUGILA) towards a reconfigurable optical add/drop multiplexer (ROADM).